This thesis examines what advising the state entails in the context of international law, through a close analysis of the role of the government legal adviser. It responds to the prevalent representations of the advisory function offered by Anglo-American practitioners, with modes of knowledge production that frequently privilege existing professional routines. The thesis develops a framework for evaluating and critiquing government advisory practice in international law, drawing on philosophical legal ethics. Philosophical legal ethics has the conceptual resources to evaluate the normative significance of professional role, and subject the values immanent in legal practice to critique. The thesis considers how the core principles in the legal ethics lexicon – the ‘adversary system excuse’, partisanship, neutrality, and non-accountability – can be modified to reflect the distinct practice context of government legal advice on international law. First, a myopic focus on adversarial adjudication gives rise to a distorted picture of international legal practice, where the legal adviser rather than a centralised decision-maker may have the latitude to influence compliance with international law. Secondly, unlike the determinate client identity required by the principle of partisanship, the competing allegiances facing legal advisers necessitate a broader engagement with questions of representation, sovereignty and the national interest. Thirdly, the principle of neutrality presupposes a level of professional independence that may be unavailable to legal advisers, given the constraints of bureaucratic structure and organisational culture. Fourthly, a principle of non-accountability facilitates the legal adviser’s disavowal of responsibility for policy outcomes, based on a decisional division of labour that restricts the scope of moral attention. After a systematic engagement with the complexities of international professional responsibility, the thesis advances a principle of interpretive responsibility to underpin the legal adviser’s invocation and construction of international law. Interpretive responsibility acknowledges the conditions that affect the transmission and reception of legal advice, while foregrounding the legal adviser’s distinct ability to shape and determine how international law frames world politics.